1. Field of the Invention
The invention is primarily concerned with method and apparatus for making magnetic recording media using gravure coating to apply magnetizable layers. The invention is specifically concerned with method and apparatus for smoothing gravure and other patterned coatings.
2. Description of the Related Art
The magnetizable layers of most magnetic recording media consist of coatings of fine magnetizable particles in organic binder. The magnetizable particles typically have a maximum dimension of less than 1 .mu.m. A magnetizable coating should be of uniform thickness, typically less than 1.0 mil (25 .mu.m), and should be free from defects such as pinholes, streaks, and particle agglomerates. The coatings can be applied by a direct gravure coater as illustrated in U.S. Pat. No. 3,761,311 (Perrington et al.), FIG. 1 of which shows "a tank 10 which is continuously supplied with a dispersion 11 of magnetizable particles and binder. This is picked up in the fine grooves of a gravure roll 12 which is scraped by a doctor blade 13 so that substantially the only material left is that contained in the grooves. The dispersion is pressed by a rubber roll 14 into contact with and transferred to an uncoated backing member 15 which is moving at the same speed and in the same direction as the gravure roll 12, as indicated by the arrow 16. Before significant evaporation of the volatile vehicle, the knurl pattern of the coating is smoothed out by a flexible blade 17. The coated backing member then passes between a pair of bar magnets 18 to physically align the magnetizable particles and on to a heated oven 19 to dry the coating" (col. 4, line 65 through col. 5, line 4).
The Perrington patent does not explain the smoothing operation of the flexible blade 17 or give any information as to its construction.
Japanese Pat. No. 50-22835, which issued Mar. 11, 1975 to Fuji Film Co., concerns the smoothing of wet coatings of magnetizable particles that have been applied by gravure coating. It reports testing of two types smoothing sheets of plastic film, the working or trailing edges of which are (1) curved or (2) straight except for diagonal corners. Both types afforded improved smoothing compared to a prior rectangular sheet. In the first of two experiments, the smoothing sheets were poly(ethyleneterephthalate) having a thickness of 10 .mu.m, a width of 30 cm, and a length of 12 cm. In the second experiment, the smoothing sheets were triethlycellulose having a thickness of 20 .mu.m, a width of 30 cm, and a length of 18 cm. The Japanese patent '835 also concludes that a thinner smoothing sheet is more effective than a thick film, because a thinner smoothing sheet makes better contact with the coated backing member.
U.S. Pat. No. 4,547,393 (Asai et al., assigned to Fuji Photo Film Co.) cites Japanese patent publications No. 96469/72 and 53631/74 for prior teachings of using a flexible sheet to smooth a coating of highly viscous solution. In the Asai patent, a flexible sheet smooths while the coating is subjected to a magnetic field. The Asai patent says: "The thickness of the flexible sheet is from about 4 microns to 120 microns. The material for the sheet includes a polyester film comprising polyethylene terephthalate; a cellulose derivative film comprising cellulose triacetate and cellulose diacetate; a polycarbonate film, a polyimide film; a polyolefin film comprising of polyethylene, polypropylene; a polyvinyl chloride film and a polyvinylidene chloride film" (col. 3, lines 45-52). In FIG. 1 of Asai, the flexible smoothing sheet 4 is mounted on a pedestal 5. "Longitudinal lines or streaks formed on sheet 4 can be prevented by moving the position of fixing pedestal 5" (col. 3, lines 15-17). In FIG. 2, the flexible smoothing sheet 11 is mounted on a roll 12. "The contact area of the sheet at the smoothening area can be adjusted by turning roll 12 in the direction of the arrow shown in FIG. 2. Longitudinal lines or streaks occurring on a sheet can be prevented by adjusting the contact area" (col. 3, lines 28-32).
U.S. Pat. No. 4,870,920 (Kageyama et al.) uses a rigid smoothing bar to smooth wet coatings of magnetizable particles and cites Japanese Patent O.P.I. Publication No. 8646/1979 for "using a bar shaped stiff member having a smooth surface called a smoothing bar or such a process using a deformable plate-shaped stiff member having a smooth surface called a smoothing blade" (col. 1, lines 33-38). It then cites Japanese Utility Model O.P.I. Publication No. 6008/1980 for "a process using a combination of the described flexible sheet and a bar" (col. 1, lines 38-41). In that combination, "a bar provided with a flexible sheet is reciprocated in the direction perpendicularly intersecting to the traveling direction of a support" (col. 1, lines 60-63). The smoothing bar of the Kageyama patent is formed with grooves 7 that move the coating laterally without needing to reciprocate the smoothing bar.
Japanese Pat. publication No. Shouwa 60[1985]-57387 assigned to Fuji Film K. K., uses a solid body smoother 3 (FIG. 1) to smooth wet coatings of magnetizable particles. The solid body smoother can be formed of stainless steel to have a shape such as those shown in FIGS. 2 and 3 of the '387 publication.